Ask someone to picture a defibrillator and most people describe two very different objects without realising it. One is a small device fitted permanently inside a patient’s chest. The other is the bright yellow case mounted on a wall outside a station, gym, or community centre. Both are correctly called defibrillators, and confusing the two is not just a semantic slip — it can make a bystander hesitate at exactly the moment hesitation is most costly. This article sets out what an implantable cardioverter defibrillator (ICD) actually does, how it differs from the public-access automated external defibrillator (AED) that sits inside The Circuit’s national network, and what current UK resuscitation guidance says about using one when the other is already present.
The Implanted Device: An ICD’s Job Is Prevention, Not Rescue
An implantable cardioverter defibrillator is a small, battery-powered device that a cardiologist surgically implants inside a patient’s chest, typically near the collarbone, with electrode leads running into the heart. Once fitted, it monitors the heart’s rhythm continuously, around the clock, with no action required from the patient. If it detects a life-threatening arrhythmia — ventricular tachycardia or ventricular fibrillation, the same rhythms that cause sudden cardiac arrest — it delivers a controlled internal shock automatically, without anyone else present or aware. ICDs are fitted in people already known to be at high risk of sudden cardiac death, usually following a prior cardiac event or a diagnosed heart condition, and their purpose is long-term, silent protection rather than emergency rescue by a third party.
The Public-Access AED: A Bystander’s Tool for a Stranger’s Emergency
A public-access AED is the opposite kind of device in almost every respect. It is external, portable, and designed to be picked up and used by someone with no medical training at all, on a person they have often never met before. Rather than monitoring continuously, it only analyses a heart rhythm when a rescuer applies its pads to someone in cardiac arrest. It then talks the rescuer through the process with voice and visual prompts, and only advises a shock if the rhythm it detects is genuinely shockable. This is the device found in the wall-mounted cabinets and lockable boxes referenced across this site’s coverage of public-access defibrillator infrastructure, and it is the category of equipment that community registries such as The Circuit exist to map and keep rescue-ready. Where an ICD prevents a known patient’s own heart from failing silently, an AED responds to an emergency in a stranger, often a first responder’s only tool before an ambulance arrives.
ICD vs AED at a Glance
| Feature | Implantable Cardioverter Defibrillator (ICD) | Public-Access AED |
|---|---|---|
| Placement | Surgically implanted inside the chest | External, applied to the chest by a rescuer |
| Monitoring | Continuous, 24/7 | On-demand, only when applied |
| Who acts | No bystander involved — fully automatic | A bystander or first responder, guided by voice prompts |
| Typical user | A single known patient, already diagnosed | Anyone nearby during a cardiac arrest emergency |
| Purpose | Long-term prevention of sudden cardiac death | Emergency treatment of an arrest already underway |
Can You Use a Public AED on Someone Who Already Has an ICD Fitted?
This is where the confusion becomes practically important. A bystander who spots a scar or a small lump below someone’s collarbone during a resuscitation attempt may wrongly assume the person is already "protected" and that using an AED is unnecessary, unsafe, or somehow redundant. Current UK guidance is unambiguous on this point: it is safe, and it is the correct action. The Resuscitation Council UK’s Adult Advanced Life Support Guidelines direct rescuers, where an implanted pacemaker or ICD is identified, to place the AED pad more than roughly eight centimetres away from the device, or to use an alternative pad position, rather than to withhold treatment altogether. The Resuscitation Council UK also collaborates with the British Cardiovascular Society and the National Council for Palliative Care on wider guidance covering how these implanted devices should be managed during resuscitation. The underlying message from all of it is the same: an implanted device may not have caught the rhythm in time, or may not be able to intervene in every situation, and hesitation from a bystander is a far greater risk than a correctly positioned shock.
Why the Distinction Matters for Public Health Planning
For a resource concerned with London’s health infrastructure, the practical stakes of this confusion sit alongside the physical infrastructure itself. A borough can install and register every AED it owns on The Circuit, train volunteers, and maintain cabinets to the highest standard, and still lose critical seconds if bystanders misunderstand what they are looking at when they see a scar or a device outline on someone’s chest. Public education about the difference between a device that already lives inside a person and one that a stranger must retrieve, open, and apply is not a footnote to defibrillator provision — it is part of what makes that provision actually work in the moment it is needed. Clear, simple explanation of "which defibrillator is which" belongs in the same conversation as siting, signage, and registry coverage, because all of it is aimed at the same few minutes between collapse and shock.
Conclusion
An ICD and a public-access AED share a name and a general purpose — correcting a dangerous heart rhythm with an electrical shock — but almost nothing else. One is a permanent, silent safeguard fitted inside a known patient; the other is a public rescue tool meant to be used, without hesitation, by whoever happens to be nearby. Understanding the AED vs ICD distinction is not a technicality for clinicians alone. For anyone who might one day stand over a stranger in cardiac arrest, knowing that the presence of one device is never a reason to withhold the other could be the difference that matters.